Obesity is a chronic metabolic disorder characterized by an excess of body fat. Obesity results from prolonged positive energy balance (i.e. energy intake exceeding energy expenditure). Although the cause of the excessive positive energy balance in obesity has not been clearly defined, key components reside in the hypothalamus, specifically in the arcuate nucleus. The arcuate nucleus (ARC) of the hypothalamus is critical for regulation of energy balance and is considered to play a key integrative role between the initial afferent signals from the periphery and CNS responses. The hypothalamic melanocortinergic system, composed of proopiomelanocortin (POMC) and agouti-related peptide (AgRP) neurons located within the ARC and melanocortin receptor type 4 (MC4R)-expressing neurons throughout the neuraxis, is a major regulator of energy homeostasis. Anorexigenic POMC neurons of the ARC respond to circulating signals and contribute to the regulation of energy expenditure by releasing the anorexigenic melanocyte-stimulating hormones. Recent studies have demonstrated that POMC neurons can be divided, at least, into two subpopulations in terms of the neurotransmitter phenotype and the expression of receptors, including leptin and insulin receptors. We hypothesize that these phenotypic distinctions reflect important functional differences and that it is the interplay between two phenotypically distinct populations of POMC neurons that is required for integration of peripheral and central signaling molecules, thus controlling the anorexigenic outcome of POMC neurons. In this proposal, we will determine how novel interactions between distinct populations of POMC neurons contribute to the control of hypothalamic neurophysiology and the regulation of energy homeostasis. We will provide an entirely novel perspective on the cellular mechanisms regulating energy balance in 3 specific aims. Aim 1: Determine the extent of POMC neuron heterogeneity in the ARC. Aim 2: Determine the physiological interactions between two of the distinct subsets of POMC neurons. Aim 3: Determine the network effect of POMC neuron heterogeneity and interaction. The information that we will thereby obtain will provide novel insight into the physiological consequences of POMC-POMC neuron interactions. Our studies will also lend support to the idea that the establishment of POMC heterogeneity during early stages of hypothalamic development is a critical factor for overall energy balance. Hence the proposed studies will provide novel insights into the neurobiology of melanocortin system in general and its specific role in the control of energy balance in particular.